1. Field of the Invention
This invention relates to processing signals received from a communications channel. More particularly this invention relates to an integrated signal processing system for receiving signals suitable for use in the transmission of video, "hi-fi" audio, images or other high bit rate signals.
2. Description of the Related Art
Encoded transmission of inherently analog signals is increasingly practiced today as a result of advances in signal processing techniques that have increased the bit rate achievable in a channel. At the same time new data compression techniques have tended to reduce the bandwidth required to acceptably represent analog information. The Art is presently striving to more efficiently transmit video and audio data in applications such as cable television using digital techniques.
Various modulation techniques have been employed in digital communications. For example, quadrature amplitude modulation (QAM) is a relatively sophisticated technique favored by practitioners of digital communications. This method involves two separate symbol streams, each stream modulating one of two carriers in quadrature. QAM is particularly useful in applications having a low signal-to-noise ratio. Multilevel QAM formats are also used to achieve spectral efficiency in applications having high signal-to-noise ratios. For example, 64- and 256-QAM can achieve spectral efficiencies of 5-7 bits/sec-Hz in cable television networks.
Quadrature Phase Shift Keying (QPSK) is a special case of the general Quadrature Amplitude Modulation (QAM) scheme, useful in applications having a low signal-to-noise ratio.
The ITU-T have adopted the DVB QPSK modulation scheme as the international standard for direct to home digital satellite broadcasting. In Europe 16-QAM and 64-QAM are used in the DVB standard for digital cable broadcasting. In QPSK a signal constellation consisting of 4 symbols is transmitted, each having a different phase and a constant amplitude. The scheme is implemented as the sort of orthogonal components, represented by the equation. EQU A.sub.m =be.sup.j.theta..sbsp.m
where .theta..sub.m can be any of {0, .pi./2, .pi., 3.pi./2}. It is necessary to transmit both sidebands in order to preserve the quadrature information.